The invention relates to a method for determining the actual torque developed by an internal combustion engine by evaluating the trace of the rotational speed (rpm) of the crankshaft of the engine.
It is known to provide a transducer disc or a transducer wheel having markings on the crankshaft of an internal combustion engine for detecting the position of the crankshaft. The transducer disc or transducer wheel is scanned by a fixed pickup. The transducer wheel is configured, for example, as a transducer toothed wheel having teeth as markings on the periphery of the transducer wheel. The pickup is configured, for example, as an inductive pickup wherein voltage pulses are induced by the teeth which run past with a rotation of the crankshaft and of the transducer toothed wheel. The time-dependent spacings of the voltage pulses or of the teeth of the transducer toothed wheel (the so-called tooth times) are measured. From the measured tooth times, the trace of the rpm of the crankshaft is then determined. In a subsequent evaluation step, the trace of the actual torque, which is developed by the engine, is determined from the trace of the rpm.
The actual torque of the engine is transmitted to a central control unit of the engine for optimizing the power characteristic, the noise performance and the exhaust gas performance of the engine. In conventional internal combustion engines having intake manifold injection, the air charge of the cylinders of the engine is measured for determining the actual torque developed by the engine. The actual torque can, however, be determined only in conventional engines which are operated with lambda=1 (wherein the air/fuel mixture is in the ratio of 1:1). In internal combustion engines of newer types and especially for so-called lean engines and stratified charge engines (direct injecting internal combustion engines; gasoline or diesel engines), which are operated up to lambda=10 (air/fuel mixture in the ratio of 10:1), the air charge of the cylinders remains constant and only the fuel quantity, which is injected into the cylinders, is varied. Therefore, the actual torque of the engine cannot be determined for the air charge of the cylinder in engines of the newer type. For this reason, the actual torque is determined, for example, from the rpm of the crankshaft of the engine.
It is an object of the invention to provide a method of the above-mentioned kind which is so configured and improved that it operates simply and quickly and especially requires little computing power.
The method of the invention is for determining the actual torque (M_act) developed by an internal combustion engine by evaluating the trace of the rpm (n) of the crankshaft of the engine. The method includes the steps: determining the mean rpm (n_) from a trace of the rpm (n) in a work stroke of a piston of the engine; determining a first area (F1) between the trace of the rpm (n) in the first half of the work stroke and the mean rpm (n13 ); determining a second area (F2) between the trace of the rpm (n) in the second half of the work stroke and the mean rpm (n13 ); determining an index (A) for the actual torque (M_act) from one of: the difference of the first area (F1) and the second area (F2) or from the ratio of the first area (F1) to the second area (F2); and, determining the actual torque (M_act) developed by the engine from the index (A) for the actual torque (M_act).
In the method according to the invention, preferably an inductive rpm transducer is utilized. The rpm transducer includes, on the one hand, a transducer toothed wheel, which is assigned to the crankshaft of the engine, and, on the other hand, a fixed inductive pickup. Voltage pulses are induced in the pickup by the teeth which run by during a rotation of the crankshaft and of the transducer toothed wheel. The time-dependent intervals of the voltage pulses (that is, the teeth of the transducer toothed wheel), the so-called tooth times, are measured. The trace of the rpm of the crankshaft is then determined from the tooth times and the total number of teeth of the transducer toothed wheel. The trace of the rpm is plotted against the teeth of the transducer toothed wheel.
In the method of the invention, the rotational irregularity of the crankshaft is determined from the areas between the rpm trace and a mean rpm and is applied to determine the actual torque developed by the engine. In this way, an index for the actual torque can be determined in a simple manner and without a great complexity as to computations. This index is then converted into the actual torque developed by the engine. The actual torque can be determined with high accuracy from the area between the rpm trace and the mean rpm.
Each of the cylinders of the engine executes a work stroke during a two-time rotation of the crankshaft. Accordingly, the number of the teeth of the transducer toothed wheel included in a work stroke of a cylinder results from the quotient of the two-fold tooth number of the transducer toothed wheel and the number of the cylinders of the engine. When utilizing a 60-2 toothed wheel in a twelve cylinder engine, the result is that the work stroke of a cylinder includes ten teeth (2*60 teeth/12 cylinders=10). A 60-2 toothed wheel has 60 teeth on its periphery and two teeth thereof are only imaginary, that is, they are not actually configured and define gaps.
In the method of the invention, the difference (that is, the ratio of the first and the second areas in the first and second halves of the work strokes) are applied as characteristic variable. In the above-mentioned example, the first half of the work stroke therefore corresponds to five teeth of the transducer toothed wheel and the second half of the work stroke corresponds to the next five teeth. Starting from the first tooth of the transducer toothed wheel, the first half of the work stroke includes therefore the teeth 1 to 5 and the second half of the work stroke includes the teeth 6 to 10. Starting from any desired tooth of the transducer toothed wheel, the work stroke can include any desired ten teeth. The first area and the second area can include a desired interval within the five teeth of the transducer toothed wheel. However, it is important that, in the determination of the index of the actual torque, for all work strokes, the same intervals are applied for the first area and the second area.
According to a preferred embodiment of the invention, it is suggested that the first area include the entire area between the trace of the rpm in the first half of the work stroke and the mean rpm. The first area is therefore determined in the complete interval of half the work stroke. In the above example, the interval, in which the first area is determined, includes all five teeth of the first half of the work stroke.
According to another preferred embodiment of the invention, it is proposed that the first area includes the area between the trace of the rpm in the first half of the work stroke and the mean rpm within one interval. The interval can have any desired magnitude within the particular half of the work stroke.
Likewise, and in accordance with another preferred embodiment of the invention, it is proposed that the second area include the total area between the trace of the rpm in the second half of the work stroke and the mean rpm.
Likewise, it is suggested in accordance with another preferred embodiment of the invention that the second area include the area between the trace of the rpm in the second half of the work stroke and the mean rpm within an interval.
The second embodiment of the method of the invention includes the steps of: determining a first extreme (E1) of the maximum rpm (n) in a work stroke of a cylinder of the engine; determining a second extreme (E2) of the minimum rpm (n) in a work stroke; determining an index (A) for the actual torque (M_act) from one of the difference of the first extreme (E1) and the second extreme (E2) or the ratio of the first extreme (E1) and the second extreme (E2); and, determining the actual torque (M_act) developed by the engine from the index (A) for the actual torque (M_act).
The third embodiment of the method of the invention includes the steps of: determining the mean rpm (n13 ) from the trace of the rpm (n) in a work stroke of a cylinder of the engine; determining a first extreme (E1) of the maximum rpm (n) in the work stroke; determining a second extreme (E2) of the minimum rpm (n) in the work stroke; determining an index (A) for the actual torque (M_act) from one of the difference of the first and second extremes (E1, E2) and the mean rpm (n_) or from the ratio of the first and second extremes (E1, E2) to the mean rpm (n_); and, determining the actual torque (M_act) developed by the engine from the index (A) for the actual torque (M_act).
According to these two solutions, it is not areas of the rpm traces but the extremes of the rpm traces in a work stroke which are applied for determining the actual torque of an engine. The extremes of the rpm trace permit a rapid and precise determination of the actual torque in a simple manner.
According to a further preferred embodiment of the invention, it is proposed that the trace of the rpm is corrected with respect to known influence quantities (especially with respect to oscillating masses) before the evaluation of the rpm.
According to an advantageous embodiment of the invention, it is proposed that the actual torque, which is developed by the engine, is determined from the product of the index for the actual torque and an rpm-dependent characteristic line summed with an rpm-dependent offset value. The actual value thereby results from the equation:
M_act=A*kxe2x80x94p(n)+offset (n)xe2x80x83xe2x80x83(1)
wherein: M_act is the actual torque, A is the index for the actual torque and k_p(n) is an rpm-dependent characteristic line.
Advantageously, the rpm-dependent characteristic line is determined before the actual determination of the actual torque, which is developed by the engine, from the quotient of an actual torque and the determined index for the actual torque at different rpms with an internal combustion engine which exhibits the smallest possible tolerances. The actual torque is the torque actually outputted by the engine. By solving a linear equation system passing through two measurement points at each rpm, the rpm-dependent characteristic line thereby results from the equation:
kxe2x80x94p(n)=(Mxe2x88x92offset (n))/Axe2x80x83xe2x80x83(2)
wherein: k_p(n) is the rpm-dependent characteristic line, M is the actual torque outputted by the engine and A is the determined index for the actual torque developed by the engine. Accordingly, an engine is operated actually or simulatively at different rpms. The actual torque, which is outputted by the engine, is measured and divided by the determined index for the actual torque. As a result of this division, one obtains the characteristic line, which is dependent upon that rpm at which the engine is just then operated.
The rpm-dependent characteristic line is determined in advance of carrying out the method to determine the actual torque developed by the engine and is stored in a suitable manner. Access can be made to the stored characteristic line during the determination of the actual torque. The determination of the rpm-dependent characteristic line must take place for each type of engine. The determined characteristic line can then be used for all engines of this type.
It is conceivable to obtain the rpm-dependent characteristic line and the rpm-dependent offset value by simulation. Preferably, the rpm-dependent characteristic line and/or the rpm-dependent offset value are, however, determined empirically on an engine test stand. On the one hand, on an engine test stand, a realistic and practical measurement result can be obtained in which also such factors are considered which do not usually flow into a simulation. On the other hand, on a test stand, the disturbance factors, which operate on an engine, especially tolerances, can be reduced or their effect on the measurement result can be compensated.
According to another advantageous embodiment of the invention, the tolerances of the engine also flow into the determination of the actual torque developed by the engine. It is proposed that the actual torque, which is developed by the engine, is determined from the sum of the index for the actual torque and a tolerance-dependent compensation value divided by the rpm multiplied by the rpm-dependent characteristic line and finally summed with the rpm-dependent offset value. In this way, the actual torque results from the equation:
M_act=kxe2x80x94p(n)*(Yxe2x80x94T/n+A)+offset (n)xe2x80x83xe2x80x83(3)
wherein M_act is the actual torque, A is the index for the actual torque, k_p(n) is the rpm-dependent characteristic line, Y_T is the tolerance-dependent compensation value and (n) is the rpm of the crankshaft.
The tolerance-dependent compensation value is advantageously determined from the lowpass filtered difference of actual torque and desired torque multiplied by the rpm and divided by the rpm-dependent characteristic line. The tolerance-dependent compensation value then results from the equation:
Yxe2x80x94T=Lowpass {(M_actxe2x88x92M_des)*n/kxe2x80x94p(n)}xe2x80x83xe2x80x83(4)
wherein: M_act is the actual torque, M_des is the desired torque, (n) is the rpm of the crankshaft and k_p(n) is the rpm-dependent characteristic line. For the actual torque in the equation, either the actual torque from equation (1) or from equation (3) can be used or a corrected actual torque according to equation (5). The tolerance-dependent compensation value must be determined for each individual engine. This can either take place after manufacture of the engine (installation of the control of the engine) or in advance of each taking-into-service of the engine (calibration of the control of the engine) or at an inspection of the engine in a defined operating point.
According to a preferred embodiment of the invention, it is proposed that the tolerance-dependent compensation value be determined during the operation of the engine. In this way, the control of the engine can be continuously adapted to an input signal, which changes slowly, that is, the index for the actual torque and the output signal (that is, the actual torque M_act which is developed by the engine) can be correspondingly corrected.
The tolerance-dependent compensation value is adapted in specific operating ranges with a large time constant. The compensation value is lowpass filtered for this purpose. Because of the large time constant, rapid changes of the input signal do not flow or flow only in a very slight amount into the computation of the compensation value. The slow changes of the input signal especially flow into the computation of the compensation value. Such slow changes of the input signal are caused by: tolerances of the engine, deterioration (lower compression, lower friction) or in temperature deformations of the engine.
Advantageously, the tolerance-dependent compensation value is determined at a high rpm. At high rpms, which lie in the upper rpm range of the engine, the tolerances of the teeth (for example, dimensions of the teeth, pitches of tooth flanks) of the transducer toothed wheel, which is attached to the crankshaft, can be especially well corrected. The times of the change of the input signal are very low because of the toothed tolerances at high rpms. For this reason, the signals have little or no influence on the tolerance-dependent compensation value because of the lowpass filtering.
The rpm-dependent characteristic line was determined at a specific air/fuel mixture charge in a cylinder. For determining the actual torque for a lean engine, the rpm-dependent characteristic line is preferably corrected to the air/fuel mixture charge=0. For this purpose, it is proposed in accordance with another preferred embodiment of the invention that the rpm-dependent characteristic line and the rpm-dependent offset value be determined with a constant throttling of the engine and that the determined actual torque is corrected by the difference to the actual throttling multiplied by a proportionality factor. The equation for determining the corrected actual torque with an rpm-dependent characteristic line corrected to the air/fuel mixture=0 is as follows:
M_actxe2x80x2=kxe2x80x94p(n)*(A+Yxe2x80x94T/n)+offset (n)xe2x88x92yxe2x80x94p*pxe2x80x83xe2x80x83(5)
wherein M_actxe2x80x2 is the corrected actual torque, A is the index for the actual torque, (n) is the rpm of the crankshaft, k_p(n) is the rpm-dependent characteristic line, y_p is a proportionality factor and (p) is the air/fuel mixture in the cylinder. The proportionality factor is determined at an operating point with defined rpm and torque and from at least two different air/fuel mixtures in the cylinder in a lean engine. The air/fuel mixture in the cylinder is, for example, measured by an intake manifold pressure sensor.